In this study, we propose an ultrasensitive refractive index sensor based on tapered optical fiber couplers and assisted with the vernier effect to overcome the problem of low sensitivity of the conventional optical fiber refractive index sensors. Two passes of mode interferences between the even and odd modes, can be achieved in both x-polarization and y-polarization by utilizing the birefringence effect of the tapered optical fiber couplers, and the superposition of these two interferences results in the vernier effect. Based on theoretical analysis and numerical calculations, the difference in the group birefringence coefficient between the even and odd modes is observed to denote zero points when the width of the tapered optical fiber coupler is 1.6--3.2 μm, significantly enhancing the sensitivity of the refractive index sensor. Further, we experimentally demonstrate our theoretical results using a tapered optical fiber coupler with a width of 1.6 μm, achieving ultrahigh sensitivities of 30020.0 nm/RIU and -34402.5 nm/RIU near the refractive index of 1.333. Our sensor is easy to fabricate, compact in structure, and cost effective, and it exhibits significant application prospects in the fields of ultrasensitive biomedical detection and analytical chemistry.